4 Effective Delayed Neutron Fraction
97
4.2.2 Experimental Settings
4.2.2.1 ADS Experiments
The A-core was used for the measurement of β eff in the ADS experiments with
100 MeV protons [12]. The core comprised 25 fuel rods surrounded by polyethylene
reflectors as shown in Fig. A2.1. Each fuel rod (1/8
p60EUEU) was made up of an
HEU (2
× 2
and 1/16
thick) and a polyethylene moderator (PE; 2
× 2
and 1/8
thick) in an Al sheath 54 × 54 × 1524 mm as shown in Fig. A2.2. The core spectrum
was a relatively hard one with an H/U (hydrogen/uranium) ratio of approximately
50 in a thermal reactor.
A proton accelerator (FFAG accelerator) was operated to inject 100 MeV protons
(beam spot 50 mm, intensity 50 pA (Cases II-1 to II-3 in Fig. A2.3a–c) and 75 pA
(Cases II-4 to II-7 in Figs. A2.3d, g) and pulsed frequency 20 Hz) onto a tungsten
target (W) (50 mm diam. and 12 mm thick) located at position (15, H; Fig. A2.1) to
generate spallation neutrons.
Time evolution according to the injection of spallation neutrons was obtained
from the signals of a BF 3 detector installed diagonally to the target at position (10,
U; Fig. A2.1a) and an optical fiber detector [3] (coated with a powdered mixture of
6 LiF (95% enrichment) for detection based on
6 Li(n, t)
4 He reactions and ZnS(Ag)
for scintillation in the core) installed at position (15–16, M-N; Fig. A2.1). The PNS
experiments were conducted for 10 min in each case to acquire neutron signals for
data analyses by the area ratio method and the Rossi-α method. The pulsed width
of the neutron source was deduced by fitting for each detector with Eq. (4.23), as
shown in Table 4.8. Here, the reason being the pulsed width of the BF 3 detector larger
than that of the optical fiber is considered as follows: the pulsed width of the source
neutrons is large during the transport into detectors; the transported source neutrons
are detected, having the width depending on the distance from the neutron source.
4.2.2.2 Subcriticality
Subcriticality was obtained by full insertion of control and safety rods, and by the
substitution of the fuel assembly for polyethylene rods, as shown in Table 4.8. The
excess reactivity and control rod worth (C1, C2, and C3) were measured by the
positive period method and the rod drop method, respectively. In Cases II-1 to II-3,
the subcriticality was experimentally deduced with the combined use of control rod
worth and its calibration curve obtained by the positive period method. Moreover, in
Cases II-4 to II-7, some of the fuel rods “F” (Fig. A2.1) were replaced by polyethylene
reflectors and configured as shown in Figs. A2.3d–f. The subcriticality in dollar
units was acquired experimentally by the extrapolate area ratio method [1]. The
subcriticality level then ranged between 1300 and 7500 pcm.
97
4.2.2 Experimental Settings
4.2.2.1 ADS Experiments
The A-core was used for the measurement of β eff in the ADS experiments with
100 MeV protons [12]. The core comprised 25 fuel rods surrounded by polyethylene
reflectors as shown in Fig. A2.1. Each fuel rod (1/8
p60EUEU) was made up of an
HEU (2
× 2
and 1/16
thick) and a polyethylene moderator (PE; 2
× 2
and 1/8
thick) in an Al sheath 54 × 54 × 1524 mm as shown in Fig. A2.2. The core spectrum
was a relatively hard one with an H/U (hydrogen/uranium) ratio of approximately
50 in a thermal reactor.
A proton accelerator (FFAG accelerator) was operated to inject 100 MeV protons
(beam spot 50 mm, intensity 50 pA (Cases II-1 to II-3 in Fig. A2.3a–c) and 75 pA
(Cases II-4 to II-7 in Figs. A2.3d, g) and pulsed frequency 20 Hz) onto a tungsten
target (W) (50 mm diam. and 12 mm thick) located at position (15, H; Fig. A2.1) to
generate spallation neutrons.
Time evolution according to the injection of spallation neutrons was obtained
from the signals of a BF 3 detector installed diagonally to the target at position (10,
U; Fig. A2.1a) and an optical fiber detector [3] (coated with a powdered mixture of
6 LiF (95% enrichment) for detection based on
6 Li(n, t)
4 He reactions and ZnS(Ag)
for scintillation in the core) installed at position (15–16, M-N; Fig. A2.1). The PNS
experiments were conducted for 10 min in each case to acquire neutron signals for
data analyses by the area ratio method and the Rossi-α method. The pulsed width
of the neutron source was deduced by fitting for each detector with Eq. (4.23), as
shown in Table 4.8. Here, the reason being the pulsed width of the BF 3 detector larger
than that of the optical fiber is considered as follows: the pulsed width of the source
neutrons is large during the transport into detectors; the transported source neutrons
are detected, having the width depending on the distance from the neutron source.
4.2.2.2 Subcriticality
Subcriticality was obtained by full insertion of control and safety rods, and by the
substitution of the fuel assembly for polyethylene rods, as shown in Table 4.8. The
excess reactivity and control rod worth (C1, C2, and C3) were measured by the
positive period method and the rod drop method, respectively. In Cases II-1 to II-3,
the subcriticality was experimentally deduced with the combined use of control rod
worth and its calibration curve obtained by the positive period method. Moreover, in
Cases II-4 to II-7, some of the fuel rods “F” (Fig. A2.1) were replaced by polyethylene
reflectors and configured as shown in Figs. A2.3d–f. The subcriticality in dollar
units was acquired experimentally by the extrapolate area ratio method [1]. The
subcriticality level then ranged between 1300 and 7500 pcm.